1. Field of the Invention
This invention relates to a reflection type sensor emitting light to an object and detecting the light reflected by the object, thereby detecting the presence of the object.
2. Description of the Prior Art
Reflection type sensors have conventionally been used to detect an object such as a semiconductor wafer. The reflection type sensors comprise a light emitting element and a light detecting element. Light is emitted from the light emitting element to an edge of the wafer while the sensor is being moved across the wafer. The light is then reflected on the wafer edge to be detected by the light detecting element. Japanese Utility Model Publication Nos. 5-38761 and 6-70240 and Japanese Patent Publication No. 1-52897-A disclose conventional reflection type sensors respectively.
The wafer is sometimes coated with an oxide film or nitrogen film both absorbing light ranging in a certain wave band. When the wafer coated with such a light-absorbing film is detected by the conventional reflection type sensor, light emitted from the light emitting element is sometimes absorbed into the wafer but not reflected thereby, whereupon the wafer cannot be detected. In view of this problem, a plurality of types of reflection type sensors have conventionally been provided so that the sensor is changed from one to another every time the wafer is changed from one type to another. This results in troublesome changes of the reflection type sensors.
Therefore, an object of the present invention is to provide a reflection type sensor which can reliably detect the object even when the object absorbs light of a specific wave band.
The present invention provides a reflection type sensor comprising a light emitting element emitting light to an object and including a plurality of LED chips disposed to be adjacent to each other and integrated into a package, and the LED chips emit light beams belonging to different wave bands. A light detecting element detects the light reflected on the object and performs a photoelectric conversion for the light beams belonging to the different wave bands, thereby delivering a light signal. A light detecting circuit determines the presence or absence of the object on the basis of the light signal delivered from the light detecting element.
According to the above-described reflection type sensor, the light emitting element emits the light beams belonging to the different wave bands. Accordingly, even when the object absorbs light of a specific wave band, light belonging to another wave band is reflected by the object to be detected by the light detecting element, which delivers the light signal. Thus, even when the object selectively absorbs the light of the specific wave band, this can reliably be detected. Moreover, since a plurality of the LED chips are disposed to be adjacent to each other and integrated into the package, a single small-sized light emitting element emits light beams belonging to a plurality of wave bands under substantially the same condition. Consequently, the aforesaid sensor can perform a more accurate detection than the one provided with a plurality of discrete light emitting elements.
In a preferred form, at least one of the LED chips emits visible light. Consequently, a position of detection can readily be obtained since a location where the light is irradiated is rendered clear.
In another preferred form, the reflection type sensor further comprises a sensor head on which a single light emitting element and two light detecting elements are mounted. In this construction, the sensor head is moved relative to the object, and the two light detecting elements are arranged at both sides of the light emitting element perpendicularly to a direction in which the sensor head is moved. The light reflected on the object is detected by the two light detecting elements located at both sides of the light emitting element respectively. Consequently, the reflected light can reliably be detected in whatever direction a reflecting face of the object is directed.